1. Field of the Invention
The present invention relates to a motor driving device configured to drive a motor such as a fan motor that forcibly circulates cold air in a freezing showcase or a refrigerator.
2. Description of the Related Art
Conventionally, in a fan motor for forcibly circulating cold air in a freezing showcase, a refrigerator, or the like, it is required to keep the rotation speed constant, to change the rotation speed depending on the situation inside the refrigerator, or to vary the rotation speed in accordance with a predetermined profile. Such control of the rotation speed is performed by a motor driving device. For a fan motor of this type, various devices have been proposed.
From the viewpoint of electric power saving and ease of control, a brushless motor is often used as a motor to be driven by the motor driving device of this type. In this case, some components of the motor driving device, such as a rectifying circuit and a drive circuit for supplying an electric current to the brushless motor, are arranged in a motor case.
Recently, in a convenience store or a supermarket, it is required to collectively or individually manage a plurality of showcases. This makes it necessary to individually control the rotation speed of fan motors provided in each of the showcases. For this reason, the motor driving device is provided with an interface through which the rotation speed can be controlled from the outside.
FIG. 7 is a block diagram showing a motor driving device having the interface through which the rotation speed can be controlled from the outside and a driving target. The driving target of a motor driving device 1 is a fan motor 2 which is a three-phase brushless motor. In the motor driving device 1, AC 100 V or AC 200 V from a commercial power source is inputted to a power supply circuit 3. The power supply circuit 3 generates a DC voltage for operation and supplies it to each unit.
In the motor driving device 1, a speed command S3 as a pulse width modulation signal is inputted from an input unit 6 to a controller 8. Further, a driver 11 receives output signals of hall elements 10A, 10B and 10C provided in the fan motor 2. The input unit 6 is an interface device for controlling the rotation speed of the fan motor 2. The pulse width modulation is a modulation technique in which a signal is transmitted by changing a ratio of the high voltage period and the low voltage period in a pulse of a constant frequency, and the pulse width modulation signal is used in various devices.
The driver 11 generates a three-phase driving signal for driving the fan motor 2 based on a driving signal from the controller 8 such that the current rotation speed obtained by using the output signals of the hall elements 10A, 10B, and 10C becomes equal to the rotation speed instructed by the speed command S3. A power module (PM) 12 drives the fan motor 2 by supplying the DC power outputted from the power supply circuit 3 to each of phases U, V and W of the fan motor 2 based on the driving signal outputted from the driver 11. Accordingly, the fan motor 2 is driven to rotate at a rotation speed corresponding to the speed command.
Further, in the motor driving device 1, the driving current of the fan motor 2 driven in this manner is detected by the driver 11. Then, the motor driving device 1 controls the drive timing of the power module 12 according to the output signals of the hall elements 10A, 10B and 10C such that the driving current does not exceed a specified reference value. In the configuration shown in FIG. 7, the hall elements 10A, 10B and 10C, the driver 11, the power module 12, the controller 8 and a portion of the power supply circuit 3, as divided by a dashed line in FIG. 7, are accommodated in a motor case of the fan motor 2.
In a case where an interface through which the rotation speed can be controlled from the outside is installed, it is necessary for the input unit 6 associated with the interface to comply with the same safety standards as safety standards required for the fan motor. More specifically, it is required for a fan motor driving system connected to a commercial power source to comply with the safety standards of Underwriters Laboratories (UL) or the like in the case of using the commercial power source. Unless a measure is taken otherwise, the input unit 6 associated with the speed control needs to satisfy the safety standards in terms of electric shocks, lightning surges, noises and so forth, which may require large-scale measures. As a result, the configuration of the motor driving system is complicated.